Supergirl 3×17 “Trinity” Season 3 Episode 17 Promo – SUPERGIRL TAKES ON ALL THREE WORLDKILLERS – Still reeling after learning about Lena’s (Katie McGrath) secret, Supergirl (Melissa Benoist) asks Guardian (Mehcad Brooks) for a favor that could alter his relationship with Lena forever. Meanwhile, with all three Worldkillers united, Supergirl and the team brace for an epic battle. Caitlin Parrish & Erica Weiss directed the episode with story by Jessica Queller and teleplay by Caitlin Parrish & Derek Simon (#317). Original airdate 5/7/2018.